1. Field of the Invention
The present invention relates to a thin flat cable for transmitting high-frequency signals, and an electronic apparatus including the flat cable.
2. Description of the Related Art
In related art, coaxial cables are typically used as high-frequency lines for transmitting high-frequency signals. A coaxial cable includes a center conductor (signal conductor) that extends in one direction (extends in the direction of signal transmission), and a shield conductor that is provided concentrically along the outer peripheral surface of the center conductor.
Recent reductions in the size and thickness of high-frequency apparatuses including mobile communications terminals have led to cases where it is not possible to secure a space for arranging a coaxial cable inside a terminal housing.
Use of flat cables as disclosed in International Publication No. WO 2011/007660 and Japanese Registered Utility Model No. 3173143 for such a terminal housing is attracting attention. Although wider than a coaxial cable, a flat cable can be made thinner, which proves particularly advantageous for cases where there is only a narrow gap inside the terminal housing.
Each of the flat cables disclosed in International Publication No. WO 2011/007660 and Japanese Registered Utility Model No. 3173143 has a triplate strip line structure as its basic structure.
Each of the flat cables disclosed in International Publication No. WO 2011/007660 and Japanese Registered Utility Model No. 3173143 includes a flat-shaped dielectric element body having flexibility and insulating property. The dielectric element body has an elongated shape that extends in a straight line. A second ground conductor is located on a second surface that is orthogonal to the thickness direction of the dielectric element body. The second ground conductor is a so-called solid conductor pattern that covers substantially the entire second surface of a base material sheet. A first ground conductor is located on a first surface of the base material sheet opposite to the second surface. The first ground conductor includes two elongated conductors extending along the longitudinal direction, at both ends of the width direction orthogonal to the longitudinal direction and the thickness direction. The two elongated conductors are connected by bridge conductors. The bridge conductors are arranged at predetermined spacings along the longitudinal direction, and extend in the width direction. As a result, the second ground conductor has an array of openings having a predetermined length that are formed along the longitudinal direction. The bridge conductors for forming the openings are generally arranged at regular spacings along the longitudinal direction.
A signal conductor having a predetermined width and a predetermined thickness is formed in the middle of the thickness direction of the dielectric element body. The signal conductor has an elongated shape that extends in a direction parallel to the elongated conductor portion of the first ground conductor and the second ground conductor. The signal conductor is formed at substantially the center of the width direction of the dielectric element body.
When the flat cable configured as described above is seen in planar view (in a direction orthogonal to the first surface and the second surface), the signal conductor is arranged in such a way that the signal conductor overlaps the first ground conductor only at the location of the bridge conductors, and lies within each of the openings in other locations.
The flat cable described above has an elongated shape that extends in a straight line. Therefore, connection terminals to be connected by the flat cable described above can be connected to each other without any problem if these connection terminals are arranged on a straight line, and if there is no obstacle on this straight line.
However, if a component or area with which contact should be avoided exists on the straight line connecting the connection terminals, it is necessary to bend or curve the flat cable at some position along its length.
Such bending or curving causes hardly any adverse effect on the transmission of an RF signal if it is possible to make the radius of curvature larger than a predetermined value in accordance with the frequency of the RF signal to be transmitted. However, in this case, a space for realizing a large radius of curvature is required, which presents a problem for a structure that is to be arranged inside a mobile communications terminal for which miniaturization is required.
On the other hand, use of a bent flat cable that is bent at a predetermined angle (for example, 90°) presents the following problem.
In the bent portion, unlike in the straight portion at either end across the bent portion, signals are not transmitted in a TEM mode. Specifically, in the bent portion, signals are transmitted in a TE mode in which the magnetic field becomes dense on the inside of the bend and the magnetic field becomes sparse on the outside of the bend. For this reason, in the bent portion, characteristic impedance tends to vary greatly depending on the positional relationship between the signal conductor and the ground conductors. Therefore, because the bent portion tends to vary easily in shape owing to manufacturing variability or the like, the characteristic impedance of the bent portion tends to vary easily, and hence the characteristic impedance of the flat cable as a whole also tends to vary easily.